jump_label, x86: Add variable length patching support

This allows the patching to to emit 2 byte JMP/NOP instruction in
addition to the 5 byte JMP/NOP we already did. This allows for more
compact code.

This code is not yet used, as we don't emit shorter code at compile
time yet.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210506194157.846870383@infradead.org